1. Field of the Invention
The invention relates to a sensing device, and more particularly, to an out-of-plane sensing device.
2. Description of the Related Art
Complementary metal oxide semiconductor (CMOS) has become the predominant technology in digital integrated circuits. This is essentially because area occupation, operating speed, energy efficiency and manufacturing costs have benefited and continue to benefit from the geometric downsizing that comes with every new generation of semiconductor manufacturing processes. In addition, the simplicity and comparatively low power dissipation of CMOS circuits have allowed for integration densities.
Microfabrication, also known as micromachining, commonly refers to the use of known semiconductor processing techniques to fabricate devices known as micro-electromechanical systems (MEMS) or micromachined devices. In general, known MEMS fabrication processes involve the sequential addition and removal of layers of material from a substrate layer through the use of film deposition and etching techniques until the desired structure has been realized. Accordingly, MEMS devices typically function under the same principles as their macroscale counterparts. MEMS devices, however, offer advantages in design, performance, and cost in comparison to their macroscale counterparts due to the decrease in scale of MEMS devices. In addition, due to batch fabrication techniques applicable to MEMS technology, significant reductions in per unit cost may be realized.
CMOS-MEMS is a technology that uses standard semiconductor process to fabricate a chip with an integration of mechanical structure and circuitry. The advantage of such technology is that the resulting chip has a stable and precise line pitch and can be fabricated in batch. Therefore, semiconductor industry has made great effort in the development of CMOS-MEMS components.
According to standard CMOS process for fabricating MEMS devices, the COMS-MEMS process can be classified into three sub-processes, i.e. pre-CMOS process, intermediate-CMOS and post-CMOS process.
According to the pre-CMOS process, a MEMS structure is first defined. An etching stop layer is then used to protect the standard CMOS components. The advantage of such design is that the CMOS components can be free from the influence of temperature and etching during the formation of the MEMS structure. A typical process is that polysilicon is first deposited to form the MEMS structure and a layer of silicon oxide is then used to cover the CMOS components. Afterward, the layer of silicon oxide is ground flush with the technique of chemical mechanical polish (CMP). After the layer of silicon oxide is ground flush, a second stage of CMOS process is performed to fabricate circuit components. Finally, the silicon oxide is etched to release elements to form the monolithic integration of the IC and MEMS components.
According to the intermediate-CMOS process, the original process parameters are varied or the original standard CMOS process is modified to add layers of material to form the desired microstructure. However, the COMS foundries usually do not allow their clients to change the process parameters. Therefore, only those with their own foundries can change the process parameters at their own choice.
According to the post-CMOS process, the structure and CMOS process are achieved simultaneously. After the CMOS process is achieved, the MEMS structure is suspended. Alternatively, the related component processes such as electroplating or etching can be carried out after the CMOS process.
In general, the conventional post CMOS-MEMS process can only achieve vertical etching and fails to etch out a horizontal channel as the gap between the upper and lower electrodes. Therefore, most of the existing capacitor-based in-plane micromachined accelerometers are fabricated with the conventional CMOS-MEMS process. However, such accelerometers of parallel vertical comb sensing electrodes can only induce a small variation of the capacitance between the electrodes and also have high residual stress. This will cause the existing micromachined accelerometers to have poor performance.
Besides, some scholars have proposed that the out-of-plane acceleration can be sensed by an unbalanced proof mass. The principle of the invention is that when the accelerometer is subjected to an out-of-plane acceleration, the unbalanced proof mass will rotate to cause a variation of voltage. The voltage variation is used to determine the out-of-plane acceleration. The method for fabricating such out-of-plane accelerometer is first to use two mask steps to define a block for anodes on the glass substrate and space for moveable elements to move about. Afterward, a further mask step is used to define and etch a structure on the silicon wafer. Finally, the silicon substrate is bonded with glass anodes to form the desired structure. This structure is easy to be fabricated and has an advantage of larger thickness that can generate larger responsive signals. However, this structure can only sense capacitance variation of single capacitor or implement a differential capacitance sensing. It will be very hard to cope with noise for such structure. Therefore, the sensitivity of the accelerometer is very susceptible to temperature and the accelerometer will have a poor resolution.
Furthermore, some scholars have also proposed an out-of-plane accelerometer fabricated by surface process and bulk process. The method is first to use trench refill process to form a sacrifice layer for electrode gap. The silicon wafer is then subjected to a bulk etching process to fully suspend the proof mass. Finally, the sacrifice layer is removed to form the accelerometer. The advantage of such accelerometer is that the use of the trench refill process can obtain a very small electrode gap and the small electrode gap can increase the variation of capacitance between electrodes and hence the sensitivity of the accelerometer. Besides, since the entire thickness of the silicon substrate is used to form the proof mass, the proof mass will exert a greater force on other components for a given acceleration. The force exerted by the proof mass will make the components to have a greater displacement and therefore the signal of variation. However, the structure of such accelerometer is hard to fabricate. The thickness of the sacrifice layer formed by trench refill process is hard to control and therefore the measurement range is not enough. The sacrifice layer is also prone to sticking to the substrate and therefore hard to be removed.
Presently, more accelerometers have been employed in the hard disks of laptop computers to detect the vibration experienced by the hard disks. When the hard disks experience a great vibration, their pickup heads will move out of the planes of the hard disks to avoid scratch the surfaces of the hard disks. Besides, the accelerometers have also been employed in video game sets. Especially, some of video games are played with multi-axes accelerometers to increase the fun and excitement of the games. Some of the cell phones are also equipped with accelerometers to enable users to play some special games.
Accordingly, there exists a need to provide an out-of-plane sensing device to solve the above-mentioned problems.